ardupilot/libraries/AP_HAL_AVR/Scheduler_Timer.cpp

#include <AP_HAL.h>
#if (CONFIG_HAL_BOARD == HAL_BOARD_APM1 || CONFIG_HAL_BOARD == HAL_BOARD_APM2)

#include <avr/io.h>
#include <avr/interrupt.h>

#include "Scheduler.h"
using namespace AP_HAL_AVR;

#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))

static volatile uint32_t timer0_overflow_count = 0;
static volatile uint32_t timer0_millis = 0;
static uint8_t timer0_fract = 0;


void AVRTimer::init() {
    // this needs to be called before setup() or some functions won't
    // work there
    sei();
 
    // set timer 0 prescale factor to 64
    // this combination is for the standard 168/328/1280/2560
    sbi(TCCR0B, CS01);
    sbi(TCCR0B, CS00);
    // enable timer 0 overflow interrupt
    sbi(TIMSK0, TOIE0);

    // timers 1 and 2 are used for phase-correct hardware pwm
    // this is better for motors as it ensures an even waveform
    // note, however, that fast pwm mode can achieve a frequency of up
    // 8 MHz (with a 16 MHz clock) at 50% duty cycle

    TCCR1B = 0;

    // set timer 1 prescale factor to 64
    sbi(TCCR1B, CS11);
    sbi(TCCR1B, CS10);
    // put timer 1 in 8-bit phase correct pwm mode
    sbi(TCCR1A, WGM10);

    sbi(TCCR3B, CS31);      // set timer 3 prescale factor to 64
    sbi(TCCR3B, CS30);
    sbi(TCCR3A, WGM30);     // put timer 3 in 8-bit phase correct pwm mode

    sbi(TCCR4B, CS41);      // set timer 4 prescale factor to 64
    sbi(TCCR4B, CS40);
    sbi(TCCR4A, WGM40);     // put timer 4 in 8-bit phase correct pwm mode

    sbi(TCCR5B, CS51);      // set timer 5 prescale factor to 64
    sbi(TCCR5B, CS50);
    sbi(TCCR5A, WGM50);     // put timer 5 in 8-bit phase correct pwm mode

    // set a2d prescale factor to 128
    // 16 MHz / 128 = 125 KHz, inside the desired 50-200 KHz range.
    // XXX: this will not work properly for other clock speeds, and
    // this code should use F_CPU to determine the prescale factor.
    sbi(ADCSRA, ADPS2);
    sbi(ADCSRA, ADPS1);
    sbi(ADCSRA, ADPS0);

    // enable a2d conversions
    sbi(ADCSRA, ADEN);

    // the bootloader connects pins 0 and 1 to the USART; disconnect them
    // here so they can be used as normal digital i/o; they will be
    // reconnected in Serial.begin()
    UCSR0B = 0;
}

#define clockCyclesPerMicrosecond() ( F_CPU / 1000000L )
#define clockCyclesToMicroseconds(a) ( ((a) * 1000L) / (F_CPU / 1000L) )

// the prescaler is set so that timer0 ticks every 64 clock cycles, and the
// the overflow handler is called every 256 ticks.
#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256))

// the whole number of milliseconds per timer0 overflow
#define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)

// the fractional number of milliseconds per timer0 overflow. we shift right
// by three to fit these numbers into a byte. (for the clock speeds we care
// about - 8 and 16 MHz - this doesn't lose precision.)
#define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) >> 3)
#define FRACT_MAX (1000 >> 3)


SIGNAL(TIMER0_OVF_vect)
{
	// copy these to local variables so they can be stored in registers
	// (volatile variables must be read from memory on every access)
	uint32_t m = timer0_millis;
	uint8_t f = timer0_fract;

	m += MILLIS_INC;
	f += FRACT_INC;
	if (f >= FRACT_MAX) {
		f -= FRACT_MAX;
		m += 1;
	}

	timer0_fract = f;
	timer0_millis = m;
	timer0_overflow_count++;
}

uint32_t AVRTimer::millis()
{
	uint32_t m;
	uint8_t oldSREG = SREG;

	// disable interrupts while we read timer0_millis or we might get an
	// inconsistent value (e.g. in the middle of a write to timer0_millis)
	cli();
	m = timer0_millis;
	SREG = oldSREG;

	return m;
}

uint32_t AVRTimer::micros() {
	uint32_t m;
    uint8_t t;
	
	uint8_t oldSREG = SREG;
	cli();

	m = timer0_overflow_count;
	t = TCNT0;
  
	if ((TIFR0 & _BV(TOV0)) && (t < 255))
		m++;

	SREG = oldSREG;
	
	return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}

/* Delay for the given number of microseconds.  Assumes a 16 MHz clock. */
void AVRTimer::delay_microseconds(uint16_t us)
{
	// for the 16 MHz clock on most Arduino boards
	// for a one-microsecond delay, simply return.  the overhead
	// of the function call yields a delay of approximately 1 1/8 us.
	if (--us == 0)
		return;

	// the following loop takes a quarter of a microsecond (4 cycles)
	// per iteration, so execute it four times for each microsecond of
	// delay requested.
	us <<= 2;

	// account for the time taken in the preceeding commands.
	us -= 2;

	// busy wait
	__asm__ __volatile__ (
		"1: sbiw %0,1" "\n\t" // 2 cycles
		"brne 1b" : "=w" (us) : "0" (us) // 2 cycles
	);
}

#endif